The present invention relates to a circuit for driving an ink-on-demand type ink jet head.
In a recording system using an ink-on-demand type ink jet head, the crest value of the ink jet control pulse is modulated or varied depending upon the level of the video signal which represents the density of an elementary area of an image reproduced. In response to the crest value of the printing pulse, the size of the ink jet or droplet may be varied so that the black areas may be represented by the large-sized ink dots, the relatively white areas may be very small ink dots and the grey areas may be represented by ink dots of varying intermediate sizes.
In order to drive the ink-on-demand type ink jet head, the high-voltage pulse is applied to a piezoelectric transducer with the mechanical output comprising, in general, a piezoelectric crystal plate and a metallic diaphragm. When applied with the high-voltage pulse, the piezoelectric transducer is deformed to pressurize an ink chamber in communication with an ink jet nozzle, whereby the ink jet is discharged through the nozzle. When the high-voltage pulse is removed, the piezoelectric transducer returns to its initial shape so that a negative pressure is produced in the ink chamber and consequently the ink flows into the ink chamber from a supply source. In this case, air is also sucked into the nozzle as the latter is in communication with the ink chamber. When air is present in the nozzle, an ink jet in a desired quantity cannot be discharged. In order to overcome this problem, a high-voltage pulse having a very sharp leading edge is applied to the piezoelectric transducer so that a high mechanical output may be produced so as to quickly build up the pressure in the ink chamber, thereby discharging the ink jet or droplet overcoming the resistance of air entrapped in the nozzle. Thereafter, the pulse voltage is gradually lowered for a relatively long time interval so that the negative pressure produced in the ink chamber may be low, thereby avoiding the suction of air into the nozzle.
Meanwhile, unless a high-voltage pulse higher than a predetermined level is applied to the ink jet head, the latter cannot discharge the ink jet or droplet or the ink drips from the nozzle resulting in the contamination of the ink jet head and a recording medium. In general the video signal varies independently of the printing cycle. As a result, when the level of the video signal varies widely during the printing cycle, the high-voltage printing pulse is distorted or clipped so that it may not have a proper amplitude and a proper width. As a consequence, prior to the completion of the printing cycle, the negative pressure builds up in the ink chamber, resulting in the discharge of insufficient ink jet or droplet, dripping of ink from the nozzle and consequently the degradation of recording.